If you are still running Windows Phone 8.1 on one of your Lumais, its time that you should know that Microsoft has put an end to the mainstream support today, July 11th. Windows Phone 8.1 was launched three years ago, with 36 months of update which ends today.
That said, your phone will not stop working. You will be able to use the phone as is, and apps which are available in the store, but there will be no updates from Microsoft for this. The last update was GDR2 in 2015, post which Microsoft announced Windows 10 Mobile which came preinstalled on Lumia 550.
The only option you have is to upgrade to Windows 10 Mobile if your device is supported aka compatible. This will get you the first major update of Windows 10 Mobile, but not the Creators Update as it was limited to very few devices.
That said here is a piece of advice for me. Don’t Upgrade if you can still keep using it, and app usage is minimum. The primary reason is that Windows Phone 8.1 is a rock solid os when it comes to performance, and chances are Windows 10 Mobile would be slow on your phone. However, the only thing that might push you to upgrade to Windows 10 Mobile will be apps. Many developers have removed their support or the app for Windows Phone 8.1
If you still love it on Windows Phone 8.1, get an Android and be happy about it. Microsoft has no plans for getting more features on Windows 10 Mobile as well. I am using it on insiders, and all we get is bug fixes, sometimes just with 2.